There are two major aims of the proposed work. The first is the elucidation of the regulatory roles of calcium and cyclic AMP as interrelated intracellular messengers in cell activation. The cellular systems under study are: a) the action of catecholamines upon cyclic nucleotide metabolism in the rat erythrocyte; b) the action of vasopressin on the isolated toad urinary bladder; c) the actions of parathyroid hormone and calcitonin upon isolated bone cells; d) the role of the calcium-dependent regulator protein in the control of calcium transport in the red cell membrane; and e) the role of calcium in excitation-secretion coupling in the exocrine pancreas in response to pancreozymin. The second study conerns the cellular basis of action of vitamin D and its biologically-active metabolites 25(OH)D3 and 1,25(OH)2D3 in the D-deficient chicken and D-deficient piglet. Particular emphasis will be placed on a study of the effect of these hormones on the structure and transport properties (for calcium and phosphate) of isolated brush border and baso-lateral membranes from intestinal mucosal cells, and the cells of the proximal renal tubule. In the latter studies dealing with 25(OH)D3 action on calcium and phosphate transport in membrane vesicles from renal brush border and baso-lateral membranes of the hog kidney, the relationship between the actions of PTH and 25(OH)D3 will also be explored.